Introduction: Of the vertebrates in the animal kingdom, sex determination is usually  a fixed characteristic in terms of life history. Interestingly, there are a few organisms  for whom sex is a plastic condition, often determined by a combination of internal and external signals. One such group of organisms which follows this trend are  the tropical teleosts: the conspicuous coloful fish inhabiting coral reefs. The majority of reef fish change sex at some point throughout their life. In fact,  reef fish that remain as the same sex for their life span (gonochoristic) are in the  minority. There are many different patterns for sex-change. Some species will begin life as  males and switch to females (protandry), and others switch from female to male  (protogyny). Further still, some will change sex in both directions, and others will be both sexes at the same time. Sex-change therefore becomes quite fascinating from several different perspectives. From the behavioral standpoint, how does a fully functional female behaviorally  become a male in a matter of hours, followed by a physiological and anatomical  change to functionally become the opposite sex. The endocrine system is most likely  responsible for this changing ability, but the hormones have yet to be identified.  Also yet to be identified are the chromosomes and genetic sequences responsible  for allowing this sexual plasticity. Heniochus monoceros, Northern Great Barrier Reef, Australia. Photo by C.L.Rice.©1999